Apparatus for generating a stepped voltage waveform and color display system utilizing same



. Jan. 27', 1970 R. 1.. WEBER 3,492,416 APPARATUS FOR GENERATING A STEPPEDVOLTAGE WAVEFORM AND COLOR DISPLAY SYSTEM UTILIZING SAME Filed April 6. 1967 3 Sheets-Sheet 1 DC 5/45 Sealer! THEEE COLOE Luv: 5EQUE/VT/AL V1050 S/A/fl 3,492,416 RM AND R. L. WEBER Jan. 27, 1970 APPARATUS FOR GENERATING A STEPPED VOLTAGE WAVEFO COLOR DISPLAY SYSTEM UTILIZING SAME Filed April 6, 1967 3 Sheets-Sheet 2 Jan. 27, 1970 I R. L. WEBER 3,492,416 APPARATUS FOR GENERATING A STEPPED VOLTAGE WAVEFORM AND COLOR DISPLAY SYSTEM UTILIZING SAME Filed April 6, 1967 3 Sheets-Sheet 5 United States Patent US. Cl. 1785.4 6 Claims ABSTRACT OF THE DISCLOSURE Apparatus is illustrated for applying three different electron accelerating voltages in sequence to the phosphor screen of a color display system. The screen includes phosphors which emit different colors of light when impinged upon by electrons of different energies. The screen comprises a capacitive electrical load and is driven through a transformer. The transformer provides an inductive reactance which is selectively resonated with the capacitive load. Respective current pulses are applied to the primary winding of the transformer through three silicon controlled rectifiers. The controlled rectifiers are turned off at the end of the respective current pulses by resonant interaction between the inductive reactance of the transformer and the capacitive load. The pulses of current change the charge on the screen thereby providing three different electron accelerating voltages.

This invention relates to apparatus for generating a three-level stepped voltage waveform and more particularly to such apparatus for applying three different electron accelerating voltages to the phosphor screen of a color display system.

Various prior art color display systems have been proposed in which the color of light produced by the phosphor screen of a kinescope is varied by varying the energy or velocity of the electrons impinging upon the screen. One proposed method of rendering phosphors differently responsive to electrons of different energies is to coat or overlay certain of the different color phosphors with a barrier layer so that only electrons having energies of at least pedetermined level will excite the phosphor. To obtain a multicolor display by this method, it is necessary to apply different accelerating voltages to the phosphor screen as the screen is scanned by an electron beam. The different voltages must be applied in proper sequence and at a relatively rapid rate, e.g., at a line-sequential rate, Since the phosphor screen in such color display systems comprises a capacitive electrical load, a relatively heavy power drain will be required of the voltage generating apparatus if the charges stored on the screen in obtaining the different accelerating voltages are dissipated during each cycle. In preferred forms of such color display systems, three different colors are employed in the display and thus three different accelerating voltages must be applied in sequence to the phosphor screen.

Among the several objects of the present invention may be noted the provision of apparatus for generating a three-level stepped voltage waveform across a capacitive load such as the phosphor screen of a color kinescope; the provision of such apparatus which can change voltage levels at a relatively rapid rate, e.g., at a line-sequential rate; the provision of such a system which does not dissipate relatively large amounts of power; the provision of such a system in which the voltage level remains substantially constant at each level of the stepped waveform; the provision of such apparatus which is highly reliable; and the provision of such apparatus which is relatively simple and inexpensive. Other object and fea- ICC ture will be in part apparent and in part pointed out hereinafter.

Briefly, apparatus according to the present invention is operative to generate a three-level stepped voltage waveform across a capacitive load. The apparatus includes a transformer having a secondary winding for connection to the capacitive load and a primary winding which is inductively coupled to the secondary winding. The transformer has a predetermined leakage inductance and predetermined losses. A first controlled rectifier means is provided for selectively connecting the primary winding across said voltage source to produce a pulse of current through the primary winding in one direction thereby to obtain a first voltage across that winding. A second controlled rectifier means selectively shunts the primary winding to produce a pulse of current in the opposite direction thereby to obtain a second voltage across the primary winding which is of opposite polarity to the first voltage and of smaller amplitude due to said transformer losses. A third controlled rectifier means, which is oppositely oriented with respect to the second rectifier means, selectively shunts the primary winding thereby to produce a second pulse of current in said one direction thereby to obtain a third voltage across the primary winding, the sum of the two said pulses in said one direction being substantially equal in magnitude to the one pulse in said opposite direction so that said third voltage is substantially equal to the initial voltage across said primary winding, Means are also provided for sequentially energizing the first, second and third rectifier means at timed intervals, each of the rectifiers being reverse biased and turned off by reasonance of the leakage inductance with the capacitive load after each respective current pulse is completed, whereby a three-level stepped voltage waveform is generated across the load.

In the accompanying drawings in which one of various possible embodiments of the invention is illustrated,

FIGURE 1 is a diagrarnmic illustration of a color display system employing a phosphor screen to which three different electron accelerating voltages are applied by apparatus according to this invention;

FIGURE 2 is a circuit diagram of an equivalent circuit of the apparatus of FIGURE 1;

FIGURE 3 is a circuit diagram of a simplified equivalent circuit of the apparatus of FIGURE 1;

FIGURE 4 illustrates graphically the flow of current to and voltage changes on the screen of FIGURE 1 in relation to current flow paths indicated on a repeated showing of the simplified equivalent circuit of FIGURE 3; and

FIGURE 5 is a schematic circuit diagram of a modification of apparatus of this invention.

Corresponding reference characters indicate corresponding parts throughout several views of the drawings.

Referring now to FIGURE 1, there is indicated at 11 a color kinescope of a type with which the present invention is useful. Kinescope 11 includes a conventional glass envelope 13 having a screen portion 15, a neck portion 17 and a bell-shaped intermediate portion 18 connecting the neck and screen portions. Coated on the inner surface of the screen portion 15 is a phosphor screen or layer 19 which includes phosphors which emit light of different colors when struck by electrons of different energies. Phosphor screen 19 may, for example, be constituted by a mixture of three different kinds of phosphor particles a first of which emits red light when energized by electrons having energies above a first, relatively low predetermined level; a second of which emits cyan light when energized by electrons having energies above a second or intermediate level; and a third of which emits blue light when energized by electrons having energies above a third, relatively high predetermined level. As screen 19 is subjected to impinging electrons of increasing energies, the three kinds of phosphors are cumulatively energized so that the screen emits red light when struck by electrons at the relatively low level; warm, substantially achromatic light when struck by electrons at the intermediate energy level; and cool, substantially achromatic light when struck by electrons at the relatively high energy level. Such unsaturated color image displays are described in greater detail in application 450,705, filed Apr. 26, 1965, and now abandoned. Images presented by such displays appear to have a relatively wide range of hues subjectively having a greater saturation than that which is actually present in the colorimetric sense. Methods of preparing phosphors useful in making such a screen are disclosed in application Ser. No. 459,582, filed May 28, 1965, and now Patent No. 3,408,223.

Over phosphor screen 19 is deposited a film 21 of aluminum which is conductive and yet is also thin enough to be substantially electron permeable. By means of film 21, suitable electron accelerating voltages may be applied to the phosphor screen 19. Aluminum film 21 also extends beyond the face portion 15 of kinescope 11 onto a preselected margin of the intermediate portion 18 of envelope 13 thereby constituting a first conductive band 23 on the inner surface of the intermediate portion.

Within the neck portion 17 of envelope 13 there is mounted a conventional electron gun 27 for emitting a beam of electrons directed toward phosphor screen 19. For the purpose of the example described herein, it is assumed that this color display system is operated in a line-sequential mode. For this purpose a three color, linesequential video signal is applied to gun 27 for varying the electron beam current, that is, the number of electrons which are emitted by the gun. The video signal thus controls the instantaneous brightness of the light produced by the beam on phosphor screen 19. It should be understood, however, that other modes of presentation, such as fieldsequential, may also be employed by appropriately varying the different voltage switching rates described hereinafter and applying a correspondingly switched video signal to gun 27.

Electrons emitted from gun 27 pass through the magnetic influence of a deflection yoke 29. Yoke 29 is energized in conventional manner to deflect the beam of electrons over the screen 19 in a scanning raster comprising a series of generally parallel horizontal lines. However, as is understood by those skilled in the art, the raster will be of uniform size only if the electrons emitted by gun 27 are all accelerated to the same energy or if compensation is made for the different deflection effects experienced by electrons having different energies.

The inner surface of the part of the intermediate envelope portion 18 adjacent neck 17 is coated with a conductive band as indicated at 33 thereby to constitute a generally annular, horn-shaped electrode which is concentric with gun 27 and through which the beam of electrons emitted by the gun pass on their way to phosphor screen 19. Band 33 may conveniently be constituted by a so-called dag coating on envelope 13. As is explained in greater detail hereinafter, electrode band 33 is employed to exercise a radial corrective effect on the deflection of the electron beam passing therethrough.

Electrical connections are made to the electrode band 33 and to the phosphor screen-covering aluminum film 21 as indicated at 35 and 37 and these connections extend through envelope 13 by means of conventional feedthrough terminals.

Screen 19 and electrode band 33 are provided with out-of-phase three-level stepped voltages of several kilovolts amplitude by the circuit indicated generally at 41. For this purpose electrode band 33 and the screen band 23 are connected to respective secondary windings W1 and W2 of a transformer TRl. The opposite ends of the transformer secondary windings are provided with respective D.C. biasing potentials by a D.C. bias source 42.

4 Appropriate nominal D.C. potentials for electrode band 33 and screen 19 are approximately 12 and 16 kilovolts, respectively.

Transformer TR1 also includes a primary winding W3, one end of which is connected to a positive D.C. supply lead L1 through a D.C. blocking capacitor C1 and the other end of which is connected to a negative D.C. supply lead L2 through the anode-cathode circuit of an SCR (silicon controlled rectifier) Q1. Triggering signals for energizing SCR Q1 are coupled to its gate terminal from a terminal 43 through a coupling capacitor C2 and the gate terminal is normally biased with respect to the cathode terminal by a resistor R1.

Primary winding W3 and capacitor C1 together are shunted by the anode-cathode circuit of second SCR Q2, the anode of which is connected to the positive supply lead L1 and the cathods of which is connected to the lower end of the primary winding. Triggering signals for SCR Q2 applied to a terminal 47 are coupled to the gatecathode circuit of SCR Q2 by a transformer TR2.

The primary winding W3 and the capacitor C1 together are also shunted by the anode-cathode circuit of a third SCR Q3. The orientation of SCR Q3 is opposite that of SCR Q2 and the anode of SCR Q3 is connected to the lower end of winding W3 and its cathode is connected to the positive supply lead L1. Triggering signals for SCR Q3 applied to a terminal 49 are coupled to the gate terminal through a coupling capacitor C3 and the gate terminal is normally biased with respect to the cathode terminal by a resistor R2.

When SCRs Q1, Q2 and Q3 are triggering sequentially, the circuit 41 operates, as described in greater detail hereinafter, to apply respective three-level stepped voltage waveforms to the electrode band 33 and to the screen 19. The transformer secondary windings W1 and W2 are connected so that the waveforms applied to screen 19 and band 33 are out-of-phase with respect to each other.

Electrons emitted by gun 27 during the different time intervals corresponding to the three different voltage levels of the waveform applied to screen 19 are accelerated to different energy levels before reaching phosphor screen 19. The respective energy levels are chosen in relation to the threshold or level-sensitive characteristics of the phOsphOl's which make up screen 19 so that the lower energy electrons excite only the red phosphor; the intermediate energy electrons excite both the red and cyan phosphors thereby causing warm, substantially achromatic light to be emitted; and the high energy electrons cause all of the phosphors, including the blue, to be energized thereby causing cool, substantially achromatic light to be emitted.

The stepped voltage waveforms are synchronized by triggering the SCRs Q1, Q2 and Q3 in timed relation to the sequence in which video signals representing different colors are applied to gun 27, so that the different accelerating voltages are produced during periods which correspond to the sequencing of the color video signal applied to gun 27. The beam current is thus modulated to reproduce the various image components in their respective colors. In the example illustrated this is assumed to be a line-sequential rate.

Registration is maintained between the different color image components substantially in the following manner which is explained in greater detail in copending application Ser. No. 553,947, filed May 31, 1966, and now Patent No. 3,439,217. During the display of a cool achromatic or bluish line, the screen 19 is driven to the highest of its three potential levels and electrons emitted from gun 27 are accelerated to a relatively high energy level. These electrons thus produce cool achromatic light when they strike the phosphor screen as explained previously. As the screen 19 is driven to its most positive voltage, the electrode band 33 is driven to its most negative voltage level. Accordingly, electrons emitted from gun 27 during this period are not greatly accelerated as they first leave the gun but rather attain only a relatively low velocity in the region of the yoke 29. For a given deflection signal, these electrons are thus relatively highly subject to deflection by the yokes field and therefore follow a path having an early high curvature as represented at A in FIGURE 1. As these electrons leave the vicinity of electrode 33, however, they are subjected to a relatively intense electric field and are thus accelerated to approach screen 19 at a relatively steep angle, impinging at a point indicated at P.

When a red line is being displayed, the screen 19 is driven to the lowest of its three voltage levels. The total acceleration experienced by electrons emitted by gun 27 during this line period is thus relatively small and only the red phosphor is energized. While the screen is at its low voltage level, the electrode band 33 is driven to the highest of its three voltage levels and thus electrons emitted from gun 27 are rapidly accelerated as they first leave the gun. These electrons are thus not greatly deflected in the region of the yoke and therefore follow a path substantially as indicated at B in FIGURE 1. However, as screen 19 is then at the lowest of its two voltage levels, these electrons are not greatly further accelerated before reaching the screen and therefore approach the screen substantially at the angle determined by their earlier deflection, striking the screen substantially at the same point P as the higher energy electrons following the path A.

When a line is to be displayed in warm achromatic light, both the screen 19 and the electrode band are maintained at their respective intermediate voltages and the electrons emitted by gun 27 follow a generally intermediate path as indicated at C and again strike the screen at the same point P.

Since the operation of switching circuit 41 depends upon the non-ideal behavior of transformer TR1, that is, the transformer acts as an inductive reactor as well as a transformer, the operation of this circuit may conveniently be described using equivalent circuits. A fairly complete equivalent circuit is illustrated in FIGURE 2 and the significance of each of the elements thereof is indicated in Table 1.

TABLE 1 V1=Voltage provided between leads L1 and L2. RQ1=On resistance of Q1.

RQ2=On resistance of Q2.

RQ3=On resistance of Q3. CC=Capacitance of capacitor C1. CP Primary distributed shunt capacitance. RP'=D.C. resistance of primary. LLP=Primary leakage inductance. RE=Eddy-current loss.

RH=Hysteresis loss.

LP=Incremental primary inductance. LS=Incremental secondary inductance. RS=D.C. resistance of secondary. LLS=Secondary leakage inductance. CS=Secondary distributed shunt capacitance. CL=External load capacitance. CM=Transformer interwinding capacitance. VL=Load voltage.

For simplicity of explanation, however, the equivalent circuit of FIGURE 2 may be further reduced by combining the effects of certain of the elements, by eliminating others whose values cause their effect to be insignificant, and by referring the value of all components to the primary winding so that the transforming or turns ratio effect of transformer T R1 may be omitted. Such a simplified unity-turns-ratio equivalent circuit is shown in FIG- URE 3. In this simplified equivalent circuit the significance of each of the previously unidentified components is as indicated in Table 2.

TABLE 2 S1=SCR Q1. S2=SCR Q2.

LLPS=Total leakage inductance referred to the primary winding.

RT=Total of transformer losses including resistance, eddy-current and hysteresis losses, referred to the primary winding.

CLT=Tota1 loading capacitance including the external capacitive load and distributed winding capacitance, all referred to the primary winding.

FIGURE 4 includes graphs representing the voltage VL on and the instantaneous current IL through the loading capacitance CLT as a function of time, together with a repeated drawing of the simplified equivalent circuit in which the path of instantaneous current flow during each significant period of the circuits cycle of operation is shown by a heavy line.

Assuming that the loading capacitance CLT is initially charged to an initial voltage VA which is smaller than voltage V1, the triggering of SCR Q1 (corresponding to the closure of switch S1) causes the source potential V1 to apply an impulse of current to the series circuit comprising the inductive leakage reactance LLPS and the loading capacitance CLT. The sudden application of this voltage at the start of the period designated T1 causes a sinusoidally oscillatory current to start to flow in the resonant circuit comprising the inductive leakage reactance LLPS and the loading capacitance CLT. As the leakage inductance is relatively small, the natural period of this oscillatory current is quite short. The voltage across capacitance CLT thus quickly reaches a peak value higher than the source voltage V1. However, as the oscillatory current tends to reverse direction following the sinusoidal pattern, the rectifying characteristic of the SCR Q1 constituting switch S1 causes the switch to essentially become an open circuit so that current can no longer flow from the charged loading capacitance CLT through the voltage source. The triggering of SCRQl thus produces only a single pulse of current which flows in one direction during the first interval T1. During the second interval, which is designated T2, the charged capacitance CLT can discharge only relatively slowly through the primary reactance LP which essentially shunts the loading capacitance. Since this shunting reactance comprises a relatively high value of impedance, the loading capacitance CLT can discharge only relatively slowly and the voltage across the load thus remains relatively constant during this relatively long interval.

After a time interval equivalent to one line scan period, SCR Q2 is triggered. Triggering SCR Q2 has the effect of closing the switch S2 in the equivalent circuit of FIG- URE 3 thereby providing a low impedance discharge path to the loading capacitance CLT which path bypasses the voltage source. In effect, the input circuit is shorted and a pulse of current flows in the direction opposite to the pulse in the one direction described previously. The loading capacitance thus discharges during interval T3 according to a sinusoidal voltage characteristic which is centered on the potential at lead L1. The potentials at the beginning and end of the period T3 are thus of opposite polarity with respect to the voltage at lead L1 and the voltage at the end of this period is substantially smaller than the voltage at the start due to losses in the transformer. At the end of interval T3, the rectifying characteristics of SCR Q2 cause the switch S2 in the equivalent circuit to become effectively open so that the loading capacitance CLT, having been charged in the reverse direction by its resonance with the leakage reactance LLPS, can now discharge only through the relatively slow path provided by the primary inductive reactance LP. The load voltage therefore remains relatively constant during the interval T4.

After a period equal to one line sweep following the triggeringof SCR Q2, the SCR Q3 is triggered providing a low impedance discharge path for the loading capacitance CLT in the opposite direction. The discharging of capacitance CLT in this opposite direction thus produces a second pulse of current flowing in the same direc tion as the pulse of current passed when SCR Q1 was triggered. The loading capacitance CLT thus discharges according to a sinusoidal function. The losses in the transformer TRl are such that, after the sinusoidal discharge of capacitance CLT during period T5, the load voltage is returned substantially to its initial level VA, the two pulses of current in the one direction being substantially equal in magnitude to the one pulse in the opposite direction. Stated in another way, the losses in the transformer TRl determine the amount of energy remaining in the resonant system after the triggering of SCRs Q2 and Q3 and thus these losses determine also the actual value of the assumed initial voltage VA. Thus, by varying the design of the transformer, the value of the intermediate voltage level VL may be adjusted in relation to the upper and lower voltage levels. After a period equal to three line sweeps has passed, the entire cycle is repeated.

From the foregoing it can be seen that a three-level stepped waveform is repeatedly generated across the loading capacitance CLT. As is understood by those skilled in the art, this stepped voltage is substantially the same as that which appears across the primary winding of the transformer T1. Accordingly, similar three-level stepped voltage waveforms, amplified by the respective turns ratios of windings W1 and W2 with respect to winding W3, are applied to the electrode band 33 and screen 19. These stepped waveforms are superimposed upon the respective bias voltages and, as noted P eviously, are applied out-of-phase with respect to each other to provide the color display mode of operation described previously. Since the switching from level to level of the stepped waveform is obtained by means of a resonant interchange of energy between inductive and capacitive components and energy is added to the system from the source only during conduction of SCR Q1, it can be seen that energy consumption and power requirements are minimized. Further, there are no discrete dissipating resistances but rather only the distributed resistances of the other elements.

The position of the intermediate voltage level in the three-level stepped waveform may also be adjusted by putting in series with SCR Q3, in the circuit shunting the primaring winding W3, a means providing a substantially constant voltage drop. The circuit shown in FIGURE 5 incorporates such a modification. SCRs Q1 and Q2 are connected as in the embodiment of FIGURE 1. SCR Q3, however, shunts the primary winding W3 through a circuit which includes a Zener diode Z1 in series with the SCR. Zener diode Z1 provides a predetermined voltage drop during conduction of SCR Q3 and, with regard to FIGURE 4, has the effect of shifting the center voltage point of the sinusoidal discharge which occurs during the period T5, thereby shifting also the level of the third or intermediate voltage level at which this sinusoidal discharge ends. By appropriately choosing the voltage drop provided by the Zener diode Z1, the level of the intermediate step in the three-level waveform may be placed substantially half way between the upper and lower levels.

While silicon controlled rectifiers have been illustrated by way of example, it should be understood that other controlled rectifying devices having the desired turnoff characteristics which can cause a resonant circuit to be opened when the current tends to reverse may also be used.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for generating a three-level stepped voltage waveform across a capacitive load comprising:

a transformer having a secondary winding for connection to said capacitive load and a primary winding which is inductively coupled to said secondary winding, said transformer having a predetermined leakage inductance and predetermined losses;

mean providing a source of DC at substantially constant voltage;

first controlled rectifier means for selectively connecting said primary winding across said voltage source to produce a pulse of current through said primary winding in one direction thereby to obtain a first voltage across said primary winding;

second controlled rectifier means for selectively shunting said primary winding to produce a pulse of current through said pirmary winding in the opposite direction thereby to obtain a second voltage across said primary winding which is of opposite polarity to said first voltage and of smaller amplitude due to said transformer losses;

third controlled rectifier means oppositely oriented with respect to said second rectifier means for selectively shunting said primary winding thereby to produce a second pulse of current in said one direction thereby to obtain a third voltage across said primary winding, the sum of the two said pulses in said one direction being substantially equal in magnitude to the one pulse in said opposite direction so that said third voltage is substantially equal to the initial voltage across said primary winding; and

means for sequentially energizing said first, second and third rectifier means at timed intervals, each of said rectifiers being reverse biased and turned off by resonance of said leakage inductance with said capacitive load after each respective current pulse is completed, whereby a three-level stepped voltage waveform is generated across said load.

2. Apparatus as set forth in claim 1 wherein said third controlled rectifier means shunts said primary winding through a circuit which includes means providing a predetermined voltage drop.

3. Apparatus as set forth in claim 2 wherein said means providing a predetermined voltage drop comprises a Zener diode.

4. Apparatus as set forth in claim 1 wherein said controlled rectifier means are silicon controlled rectifiers.

5. Apparatus as set forth in claim 1 wherein said third controlled rectifier means is connected across said primary winding through a series circuit which includes a DC. blocking capacitor.

6. In a color display system including a kinescope having a phosphor screen which, in response to impinging electrons, emits light of different colors when different electron accelerating voltages are applied thereto, said screen comprising a capacitive electrical load, apparatus for applying three different accelerating voltages in sequence to said screen comprising:

a step-up transformer having a high voltage secondary winding one end of which is connected to said screen and the other end of which is connected to a high voltage DC. bias source, said trans-former having also a primary winding which is inductively coupled to said secondary winding, said transformer having a predetermined leakage inductance and predetermined losses;

a first silicon controlled rectifier for selectively connecting said primary winding across a voltage source to produce a pulse of current through said primary winding in one direction thereby to obtain a first voltage on said screen with respect to the voltage provided by said bias source;

a second silicon controlled rectifier for selectively shunting said primary winding to produce a pulse of current through said primary Winding in the opposite direction thereby to obtain a second voltage on said screen which is of opposite polarity to said first voltage with respect to the voltage provided by said bias source and of smaller amplitude due to said transformer losses;

third rectifier means at timed intervals, each of said rectifiers being reverse biased and turned off by resonance of said leakage inductance with said capacitive screenafter each respective current pulse is completed whereby three different accelerating voltages are applied in sequence to said screen.

References Cited a third silicon controlled rectifier oppositely oriented UNITED STATES PATENTS with respect to said second silicon controlled rectim fier for selectively shunting said primary winding to 3396233 8/1968 Kagan 178 5-4 produce a second pulse of current in san one direc ROBERT L. GRIFFIN, Primary Examiner tion, the sum of the two said pulses in said one direction being substantially equal in magnitude to ROBERT L. RICHARDSON, A i t t E i the one pulse in said opposite direction thereby to 15 return said screen substantially to its initial voltage; and 307-252; 315-30 means for sequentially energizing said first, second and US. Cl. X.R. 

